Ternary alloys

ABSTRACT

Bright, tarnish resistant and color stable ternary alloys of about 40 - 90% of tin, about 10 - 50% cobalt and about 1 - 28% of a third metal of Periodic Group II B , III B  or VI B . Typical third metals are zinc, cadmium, indium, antimony or chromium. The alloys are electrodeposited from aqueous acidic baths at a temperature of about 50 - 85° C and current density of about 5 - 45 A/ft.

BACKGROUND OF THE INVENTION

This invention relates to new and improved ternary alloys, to aqueouselectrolytic baths from which the alloys are deposited, and to a processfor forming the alloys.

Various alloys have been developed in efforts to duplicate the superiorcolor of chromium and alloys containing substantial amounts of chromium,while also providing the corrosion resistance and tarnish resistancerequired when the alloy is to be used as a protective coating.Accordingly, the prior art teaches the addition of brightening agents toplating baths for the electro-deposition of tin-nickel binary alloys, asin U.S. Pat. No. 3,141,836 --Seyb et al, or the careful control ofplating conditions, also in the deposition of nickel-tin binary alloys,such as the highly acidic baths in U.S. Pat. No. 2,926,124-- Taylor etal. In another approach cobalt-tin binary alloys have been studied withrespect to close similarities in corrosion resistance to nickel-tinalloys. Clarke et al, "An Electrodeposited Bright Tin-CobaltIntermetallic Compound, CoSn," Transactions of the Institute of MetalFinishing, 1972, Volume 50.

Despite the usefulness of such alloys from the standpoint of tarnish andcorrosion resistance, those of such alloys which initially exhibitedbrightness similar to that of chromium did not maintain the good color.Moreover, results in obtaining hardness, brightness, tarnish resistanceand color stability have not been consistent. Such properties tend to beoverly sensitive to specific process conditions and therefore aredifficult to reproduce on a commercial scale.

OBJECTS AND SUMMARY

Accordingly, an object of the invention is to provide a new and improvedalloy which not only provides a chromium-like brightness and tarnishresistance, but also provides color stability and hardness superior tothat found in any of the alloying metals individually.

Still another object of the invention is to provide new and improvedelectrolytic plating baths which are easily formulated and from whichternary alloys can be efficiently deposited on a wide variety ofsubstrates to give coatings which are hard, bright, tarnish resistantand which have good color stability.

Another object is to provide a new and improved process whereby tin,cobalt and a third metal are electrolytically co-deposited to form ahard, bright coating which is stable and highly resistant to tarnishing.

These and other objects, features and advantages of the invention willbe apparent from the description which follows.

In summary outline, the foregoing and other objects are achieved in anew and improved ternary alloy consisting essentially of about 40- 90wt. % tin, about 10- 50 wt. % cobalt and about 1- 28 wt. % of a thirdmetal selected from Periodic Group II_(B), III_(A) or VI_(B). Thirdmetals include zinc, cadmium, indium, antimony or chromium, of whichzinc, indium and chromium are preferred. The third metals may be presentin the alloy singly or in admixtures of two or more. The plating bathsof the invention are aqueous and highly acidic, and contain compoundsproviding stannous ions, cobaltous ions and ions of the third metal ormetals to be deposited. The ternary alloys are efficiently co-depositedfrom the baths at a temperature of about 50°- 85° C. and current densityof about 5- 45 A/ft.². In addition to the tarnish resistance expected inalloys containing tin and cobalt, the alloys exhibit a hardness,chromium-like brightness and color stability which make them useful ascoatings on a wide variety of substrates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ternary alloys of the invention are electrodeposited from highlyacidic, aqueous baths of pH of about 1- 3. A mineral acid is utilizedfor this purpose, such as a hydro-halide or a sulfur acid. Preferredacids are hydrochloric and fluoboric acids since such acids provideanions in common with anions of preferred compounds of the metals to bedeposited, and thus promote stability of the baths and good control ofelectrodeposition therefrom.

The metals to be deposited are present in the baths as ionic compounds,the anions of the compounds and other conditions being chosen such thatthe compounds are substantially completely soluble in the aqueousmedium. Accordingly, the compounds may be present as halides, sulfates,or otherwise but preferably the compounds will have anions common to theanions of the acid utilized to provide the high acidity. Sincehydrochloric and fluoboric acids are the preferred acids, the preferredmetal compounds will be the chlorides and fluoborates of the metals.

The metal compounds may be dispersed and dissolved in the aqueous mediumin any suitable manner with heating and agitation, as needed. Sequenceof admixture is not critical although the usual precautions with highlyacidic solutions should be exercised. However, dispersion andelectroplating are each benefited by somewhat elevated temperature ofthe bath, of the order of about 50°- 85° C.

As chlorides the following ranges of concentrations of the metalcompounds in the baths are effective:

    ______________________________________                                        cobalt chloride     about 20-400 g./l.                                        stannous chloride   about 10-100 g./l.                                        zinc chloride       about 10-175 g./l.                                        ______________________________________                                    

To the baths containing the foregoing concentrations of metal compoundsmay be added hydrochloric acid (37% solution) at a concentration ofabout 40- 150 mils./l., ammonium hydroxide (28% solution) in the rangeof about 10- 50 mls./l. and ammonium bifluoride, about 20- 400 g./l., toprovide the requisite acidity and bath stability.

When the tin compound is a fluoborate, it is preferred to use fluoboricacid in place of hydrochloric acid. The concentrations of these andother ingredients in the bath may then range as follows:

    ______________________________________                                        cobalt chloride      about 100-300 g./l.                                      stannous fluoborate (50% solution)                                                                 about 25-75 mls./l.                                      fluoboric acid       about 75-225 g./l.                                       ammonium hydroxide (28% solution)                                                                  about 25-150 mls./l.                                     zinc chloride        about 10-135 g./l.                                       ______________________________________                                    

Indium chloride as a substitute for zinc chloride preferably is utilizedat a concentration of about 5- 35 g./l. and chromium chloride as asubstitute for either of the foregoing compounds is effective at aconcentration of about 5- 55 g./l.

Other conditions of electrodeposition, including the cell form ofelectrolytic arrangement and type of substrate to be coated, control ofconcentration and rejuvenation of the baths, are well known in the artand do not require further description. For example, the well known Hullcell may be utilized. The current density preferred for efficientelectrodeposition is about 5- 45 A/ft.².

Generally, the percentage of each metal in the ternary alloy will varyin direct proportion to the concentration of each metal in the platingbath. To a lesser extent the percentage of each metal in the alloy willalso vary in accordance with electroplating conditions such astemperature, current density and pH. It is believed that the new alloyexists as Sn₂ (Co, X) or (Sn, X)₂ (Co, X) where X is the third metal.

While the resultant ternary alloys are analogous to tin-nickel andtin-cobalt with respect to tarnish resistance, the alloys exhibit notonly chromium-like brightness but also consistently good color and colorstability. Moreover, while the ternary alloys resist corrosionessentially to the same extent as chromium, they have a higherresistance than chromium to strong alkali under a superimposed anodicpotential, that is, whereas chromium will dissolve if made anodic in acaustic solution, the ternary alloys of the invention are not affected.The alloys of the invention therefore are more resistant to chlorideattack than chromium and will resist salt spray and salt water contactbetter than chromium.

The plating baths may contain auxiliary reagents for various purposes inaccordance with the understanding in the art. Among such auxiliaryreagents are ammonium chloride, gluconic acid, thiourea, fluorides suchas ammonium bifluoride, sodium fluoride and potassium titanium fluoride,and various surfactants and the like such as alkyl aryl sodiumsulfonate. Such reagents generally are useful in minor amounts, forexample, about 0.01 to about 10 grams per liter of plating bath, toobtain their known benefits.

The ternary alloys may be co-deposited electrolytically upon a widevariety of substrates, including metals such as steel, brass and zinc,as well as ceramics and plastics, in accordance with techniques wellknown in the art for coating such substrates.

The following examples of aqueous plating bath formulations andconditions of electrodeposition are intended as further illustration ofthe invention but are not necessarily limited of the scope of theinvention except as set forth in the claims. All parts and percentagesin these examples as well as in the foregoing specification are byweight unless otherwise indicated. In each example the ternary alloydeposited has an approximate composition: tin, 40- 90%; cobalt, 10- 50%;third metal, 1- 28%.

    ______________________________________                                        EXAMPLE 1                                                                     ______________________________________                                        Composition of aqueous bath                                                   Cobalt Chloride       20-400 g./l.                                            Stannous Chloride     10-100 g./l.                                            Ammonium Bifluoride   20-400 g./l.                                            Hydrochloric Acid (37%)                                                                             40-150 mls./l.                                          Ammonium Hydroxide (28%)                                                                            10-50 mls./l.                                           Zinc Chloride         15-175 g./l.                                              Plating Conditions                                                            Temperature of bath 60-80° C.                                          Current density     10-30 A/ft..sup.2                                         pH of bath           1-3                                                    ______________________________________                                        EXAMPLE 2                                                                     ______________________________________                                        Composition of aqueous bath                                                   Cobalt Chloride       20-400 g./l.                                            Stannous Chloride     10-100 g./l.                                            Ammonium Bifluoride   20-400 g./l.                                            Hydrochloric Acid (37%)                                                                             40-150 mls./l.                                          Ammonium Hydroxide (28%)                                                                            10-50 mls./l.                                           Indium Chloride        5-35 g./l.                                               Plating Conditions                                                            Temperature         60-80° C.                                          Current density     10-30 A/ft..sup.2                                         pH of bath           1-3                                                    ______________________________________                                        EXAMPLE 3                                                                     ______________________________________                                        Composition of aqueous bath                                                   Cobalt Chloride       20-400 g./l.                                            Stannous Chloride     10-100 g./l.                                            Ammonium Bifluoride   20-400 g./l.                                            Hydrochloric Acid (37%)                                                                             40-150 mls./l.                                          Ammonium Hydroxide (28%)                                                                            10-50 mls./l.                                           Chromium Chloride      5-55 g./l.                                               Plating Conditions                                                            Temperature         60-80°  C.                                         Current density     10-30 A/ft..sup.2                                         pH of bath           1-3                                                    ______________________________________                                        EXAMPLE 4                                                                     ______________________________________                                        Composition of aqueous bath                                                   Cobalt Chloride       100-300 g./l.                                           Stannous Fluoborate (50%)                                                                            25-75 mls./1.                                          Fluoboric Acid         75-225 g./l.                                           Ammonium Hydroxide (28%)                                                                             25-150 mls./l.                                         Zinc Chloride          10-g./l.                                                 Plating Conditions                                                            Temperature         50-85° C.                                          Current density      5-45 A/ft..sup.2                                         pH                   1-3                                                    ______________________________________                                        EXAMPLE 5                                                                     ______________________________________                                        Composition of aqueous bath                                                   Cobalt Chloride       100-300 g./l.                                           Stannous Fluoborate (50%)                                                                            25-75 mls./l.                                          Fluoboric Acid         75-225 g./l.                                           Ammonium Hydroxide (28%)                                                                             25-150 mls./l.                                         Chromium Chloride      10-75 g./l.                                              Plating Conditions                                                            Temperature         50-85° C.                                          Current density      5-45 A/ft..sup.2                                         pH                   1-3                                                    ______________________________________                                        EXAMPLE 6                                                                     ______________________________________                                        Composition of aqueous bath                                                   Cobalt Chloride       100-300 g./l.                                           Stannous Fluoborate (50%)                                                                            25-75 mls./l.                                          Fluoboric Acid         75-225 g./l.                                           Ammonium Hydroxide (28%)                                                                             25-150 mls./l.                                         Indium Chloride        5-35 g./l.                                               Plating Conditions                                                            Temperature         50-85° C.                                          Current density       5-45 A/ft..sup.2                                        pH                   1-3                                                    ______________________________________                                    

What is claimed is:
 1. A bright, tarnish resistant and color stableternary alloy consisting essentially of about40- 90 wt. % tin 10- 50 wt.% cobalt 1- 28 wt. % third metalwherein said third metal isantimony.Iadd., zinc .Iaddend.or a metal of Periodic Group .[.II_(B),.].III_(A) or VI_(B). .[.2. A ternary alloy as in claim 1 wherein saidthird metal is zinc, cadmium, indium, or chromium..].
 3. A ternary alloyas in claim 1 wherein said third metal is zinc, indium or chromium.